One example of a drive assistance device is presented in EP 0 945 113 A2. Such drive assistance device for a wheel chair comprises a drive motor, a driving wheel, a sensor means which is designed such that it is adapted to determine a driving force induced by hand into the driving wheel and a control unit which is designed such that the drive motor for driving the driving wheel is driven in accordance with a degree of drive assistance depending on the driving force which is induced into the driving wheel by hand.
Such wheel chairs provide the wheel chair driver with the option to drive the wheel chair manually, i.e. by hand, for instance via respective hand rims provided at the driving wheels. However, supporting this manual drive depends on respective need by one or more drive motors. In order to achieve this, the sensor means detects the respective force manually induced into the hand rim and the control unit controls the drive motor or the drive motors for driving the driving wheel in accordance with a degree of drive assistance depending on the force induced into the hand rim.
The drive systems are suited to take some of the physical strain from the wheel chair driver. The manual force induced by hand of the wheel chair driver into the driving wheel, for instance via a hand rim, results in producing torque. Additionally, an electric driving force or a corresponding driving torque of the drive motors is developed in accordance with a drive assistance to a degree depending on the manually induced force due to the control unit controlling the drive motor accordingly. These forces, the torque produced by manual operation and the torque produced from the electronic driving force or drive motors, together act as a driving force or driving torque, respectively. In this kind of operation there are therefore acting a manual driving force and the torque resulting therefrom and the torque of the driving motor in the same direction. Consequently, the magnitude of the resulting force is the combination of the manual force supplied by the driver and the force supplied by the electric motor.
As a result, a wheel chair driver provides only a part of the force necessary for locomotion and, therefore, can without major strain negotiate slopes uphill and downhill. The relation between the manually induced forces and the torques provided by the electric motor, that is the level or degree of drive assistance, can be adjustable in accordance with the specific personal needs or requirements of the wheel chair driver, and, as the case may be, can be selected in advance and preset.
When driving on a slope or ramp it is still necessary for a wheel chair driver in spite of the support of the motors to provide a higher power for locomotion than in case of driving on an even plane. Moreover, the driving speed is quite slow in such driving situations, for instance at slopes or ramps. Therefore, it may happen that in those phases where the wheel chair driver has to release the grip at the hand rim in order to reposition the hand for the next push, the wheel chair can lose so much speed that, in view of the downhill force, the wheel chair rolls back before the wheel chair driver can effect a subsequent pushing motion at the hand rim.
Moreover, it is possible that the wheel chair driver underestimates a slope and, therefore, ends up in a situation there he is no longer able to muster the required power at the hand rim in sufficient frequency in order to effect locomotion in the desired direction. As the case may be, he will be forced to make short stops between two pushing motions. This, however, involves the risk that, in view of the downhill force, the wheel chair unintendedly rolls back, contrary to the desired driving direction.